Loudspeaker construction



Nov. 26, 1963 J. 1. MONTAGU 3,112,375

LOUDSPEAKER CONSTRUCTION Filed Jan. 20, 1960 2 Sheets-Sheet 1 INVENTOR. JEAN MONTAGU BY Nov. 26, 1963 J. 1. MONTAGU 3,112,375

LOUDSPEAKER CONSTRUCTION Filed Jan. 20, 1960 2 Sheets-She et 2 j 3 i 5 j 22a 5 5 f 44 E 68 70 E /O 7 0 62 4 4 a) i I 22 Lg a LL 220a F l G. 3 F l G. 4

F l G. 5

INVENTOR. JEAN l. MONTAGU 20 3 BY v flak, Swim ATTORNEYS United States Patent C) This invention relates to an efiicient loudspeaker which may be constructed with any desirable input impedance. More pmticularly, it relates to a novel loudspeaker construction incorporating a fixed voice coil and a movable magnetic element affixed to the cone or diaphragm of the loudspeaker and disposed in the air gap of a field magnet. Current through the voice coil varies the field distribution in the air gap, causing movement of the magnetic element and diaphragm.

The output stages of audio amplifiers generally have output impedances of several hundred on up to several thousand ohms, depending on various factors such as the particular circuit configuration and whether vacuum tubes or transistors are used. In order to obtain a significant amount of low-distortion power from the amplifier, the loudspeaker load presented to it must have an impedance reasonably well matched to the amplifier impedance.

On the other hand, most loudspeakers have heretofore had irnpedances of less than 50 ohms, generally in the range of 8-16 ohms. This has necessitated the use of a transformer, an item of considerable expense in quality systems, to match the loudspeaker impedance to that of the amplifier. ,The impedance limitation in prior electro magnetic loudspeakers is due to the movable voice coil construction thereof. The voice coil is mounted on the loudspeaker diaphragm and disposed in an annular air gap of a field magnet. Current through the coil reacts with the radial magnetic field in the gap to impart an axial thrust to the coil and diaphragm.

The input impedance of a moving voice coil loudspeaker increases with the number of turns in the voice coil. However, as a practical matter, the number of turns must be kept relatively low. If a large number of turns of fine wire are used, there will be substantial dissipation due to the resistance of the coil, resulting in a loss of efliciency and also presenting a heat removal problem, since the moving coil cannot conveniently be connected to a suitable heat sink. If large diameter Wire is used, there is a problem of accommodating the coil in a narrow air gap. If the air gap is widened, the flux therein decreases, again resulting in a decrease in efilciency. Another problem associated with the use of a large number of turns of large diameter wire is the weight of the voice coil which severely reduces the medium and high frequency response of the speaker.

It has been proposed to use a fixed voice coil, acting as the primary of a transformer whose secondary is a disk or single turn loop moving in the air gap of the field magnet. ment has been found to be poor, and the response falls oil rapidly at lower frequencies.

Accordingly, it is a principal object of my invention to provide a loudspeaker which may be constructed to have any desirable input impedance and thus may be directly matched to the output stage of an amplifier without the use of an intervening transformer.

Another object of my invention is to provide a loudspeaker of the above chanacter having an efiiciency comparable to transformer-matched movable-voice coil speakers.

A further object of my invention is to provide a loudspeaker of the above type having a low mass in the driving member associated with the diaphragm.

Yet another object of my invention is to provide a However, the efliciency of this arrangeloudspeaker of the above type capable of providing faithful acoustical reproduction of electrical signals applied thereto.

A still further object of my invention is to provide a loudspeaker of the above type capable of low cost fabrication.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combinations of elements and arrangements of parts which will be exemplified in the constructions hereinafter set forth and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIGURE 1 is a sectional view of a loudspeaker incorporating the features of my invention,

FIGURE 2 is an enlarged fragmentary elevation, partly broken away, taken along line 22 of FIGURE 1,

FIGURE 3 is a fragmentary sectional view of another embodiment of a loudspeaker made according to my invention,

FIGURE 4 is a fragmentary sectional view of a third embodiment using a pair of voice coils, and

FIGURE 5 is a fragmentary sectional view of a fourth embodiment of my invention using a multiple section driving ring.

In general, I have replaced the movable voice coil in prior loudspeakers with a magnetic ring as the driving element on the diaphragm. My voice coil, which is stationary, is mounted on the frame of the loudspeaker. It produces a magnetic field which opposes the static field in some portions of the air gap and augments it in other portions, depending on the direction of voice coil current. Following well-known principles, a force is exerted on the magnetic driving member to move it in a direction providing maximum air gap flux. This direction changes when the voice coil current reverses, and thus alternating-current electrical signals applied to the coil are converted to acoustical energy by way of reciprocal motion of the magnetic member and the diaphragm driven thereby.

Since the voice coil is stationary, neither the number of turns nor the wire size has any efiect on the mechanical characteristics of the loudspeaker. Furthermore, heat generated in the coil is conveniently conducted away therefrom by the metallic frame of the loudspeaker. Thus, the impedance of the speaker may be readily varied to match the impedances of various vacuum tube and transistor amplifiers. The speaker has good efiiciency as well as a highly satisfactory acoustical reproduction of the electrical signals applied thereto. In fact, the efiiciency is considerably greater than that of movable-voice coil speakers.

As seen in FIGURE 1, a loudspeaker incorporating the principles of my invention may be provided with a conventional static field structure comprising a permanent magnet it), a yoke 12 and poles l4 and 16. {The parts l2, l4 and 16 are preferably of iron or other suitable soft magnetic material. As seen in FIGURES 1 and 2, the poles l4 and 16 and a further pole l8 define an air gap 20 for the static field structure.

Still referring to FIGURE 1, the loudspeaker includes a diaphragm 22 provided with a tubular portion 22a disposed in the air gap 29. At its outer edge, the diaphragm 22 is fastened to a conventional dish-like supporting frame 24. The supporting frame, in turn, may be made integral with an annular plate 26 extending to the pole 18 and preferabiy welded to the yoke 12 as indicated at 28.

Suspension of the diaphragm 22 is completed by spiders 3t and 32 extending between the tubular portion 22:: and the frame 24 and yoke 12, respectively. Illustratively, the spiders 341? and 32 may be of thin metal or suitably impregnated cloth or paper formed with annular corrugations. They provide a high degree of flexibility 1n the axialdirection of the air gap 25 and substantial stiffness in the radial direction transverse thereto.

The plate 26 is preferably of the same material as the yoke 12, and therefore it serves to conduct the magnetic field from the yoke to the pole 18 as efiiciently as it is conducted to the pole 14. Accordingly, the poles 13 and 14 serve as a single pole as regards the static field provided by the magnet 10. They serve as opposite poles for the field created by current through a voice coil 34 mounted in the space between the plate 26 and the yoke 12. The magnetic circuit for the field developed by current through the coil 34 includes the pole 13, the plate 26, the yoke 12 and the pole 14. Thus, the gap 36, directly between the poles 14 and 13, and the neighboring portions of the air gap 2 serve as an air gap in the magnetic circuit of the voice coil 34. However, almost the entire flux of the voice coil passes through the gap 26 rather than the gap 36.

The tubular portion 22:: of the diaphragm .22 carries a driving ring 38 suitably affixed thereto. The ring 38 is of high permeability magnetic material, and its outer periphery lies immediately adjacent to the cylinder defined by the surfaces of the poles 14 and 18 bordering the gap 2t).

The operation of the speaker of FIGURES 1 and 2 is as follows. Assume that the static field of the magnet 10 is directed radially outwardly from the pole 16 to the poles 14 and 18. If a current is passed through the voice coil 34 in such direction as to generate a magnetic field in the direction indicated by the arrows (FIGURE 1), the magnetomotive forces of the magnet 10 and the coil 34 will add along the path extending from the pole 18 to the plate 26, the yoke 12, the magnet 10 and the pole 16-. They will subtract along the path defined by the pole 14, the yoke 12, the magnet 10 and the pole 16. The magnetic field in the portion of the gap 20 between the poles 16 and 18 will therefore be greater than the field between the poles 16 and 14. In a well-known manner, the driving ring 38 will be attracted into the region having the greater field intensity, much as a piece of iron situated midway between two magnets will be attracted to the stronger magnet.

Within limitations, the force exerted on the ring 33 is independent of its distance from the resting position shown in FIGURE 1. This follows from the fact that, as the ring moves to the left under the above conditions, it linearly increases the magnetic energy in the portion of the gap 20 between the poles 16 and 1S and proportionally decreases the energy of the gap in the region between the poles 16 and 14. Therefore, the force, which is proportional to the derivative of the energy in the gap in the direction of motion, is linear and independent of the position of the ring. This linear relationship ends when the left end of the ring extends beyond the plate 26 or the right end leaves the region between the poles 14 and 16.

Similarly, if the current in the coil 34 is reversed, the ring 38 will move to the right, and the force exerted thereon is essentially independent of its position until the right edge of the ring passes beyond the pole 14 or the left end lies adjacent to the gap 36.

Thus, by the application of an alternating-current to the coil 34, the ring 38 may be made to reciprocate in the gap 20, thereby driving the diaphragm 22 and generating acoustical signals at the surface thereof.

As shown in FIGURES 1 and 2, the driving ring 38 may be a separate member affixed to the tubular portion 22a of the loudspeaker diaphragm. However, other consan t on y b 113 1- For example, it may be fabricated as an integral part of the portion 22a by impregnating the latter with iron powder or a ferromagnetic oxide. It is preferable that the ring 38 be segmented, and, as illustrated in FIGURE 2, this may be accomplished by using alternate segments 3&2 of iron and 38b of coper soldered together to provide a unitary construction. The segmented construction reduces any ofi-center forces exerted on the driving ring by the center pole 16 and outer poles 14 and 18. When the ring is exactly centered, it is in equilibrium with respect to radial magnetic forces. However, if it moves slightly off center, there is a tendency for it to keep moving until it bottoms on the pole 16 or the poles 14 and 18 so as to provide essentially a magnetic short circuit between the inner and outer poles. The attraction forces are considerably reduced by the interposition of the copper segments 38b which prevent the completion of a circumferential magnetic path through the ring 33.

The driving ring 33 should occupy as great a portion of the radial thickness of the gap 29 as possible. This increases the static magnetic flux in the gap and correspondingly increases the efficiency of the loudspeaker. Furthermore, the ring 38 should be close to the poles 14 and 18 so that the magnetic path between these poles by way of the ring has considerably less reluctance than the path across the air gap 36.

In FZGURE 3, I have illustrated an embodiment of my speaker in which the voice coil 34 is associated with inner poles of the static magnetic system. As shown therein, the magnet 10 is connected between the yoke 12 and an inner pole member 40 recessed to accommodate the voice coil and form a pair of inner poles 42 and 44. The outer pole 46 of the static field is an annular member mounted on the yoke 12. Thus, the static field extends through the gap 26 between the outer pole 46 and the inner poles 42 and 44, largely by way of the dri ing ring 38. The effect of the magnetomotive force developed between the poles 42 and 44 by the coil 34 is the same as in the construction of FIGURES 1 and 2. An advantage of the construction illustrated in FIGURE 3 is the increased radius of the gap 20 with the same over-all outer dimensions of the speaker. The circumference of the gap and the areas of the opposing faces of the pole 46 and the poles 42 and 44 are thus substantially increased. A greater magnetic flux can therefore be accommodated in the gap 20 without saturating the poles.

In the construction of FIGURE 4, a pair of driving rings has been incorporated into a structure similar to that of FIGURE 3. Thus, the permanent magnet 10 is connected between pole members 48 and 50, disposed within outer poles 52 and 54 and thereby defining gaps 56 and 58. The poles 52 and 54 are separated by a ring 60 of magnetic material, and the inner pole members are mounted on the yoke 12 by a spacer 62 of nonmagnetic material. Thus, the static magnetic field runs from the magnet 10 through the pole member 56 and the poles 64 and 66 thereof, thence across the gap 58 to the pole 54. From there it continues through the ring 69 and pole 52, across the gap 56 to the poles 68 and '70 of the member 48, and thence back to the magnet 14).

Still referring to FIGURE 4, the tubular portion 22a of the diaphragm 22 extends through both the gaps 56 and 58 and supports driving rings '72 and 74 in the gaps. Voice coils 76 and 78 are accommodated in the pole members 48 and 50. Operation of each of the voice coils on the driving ring adjacent thereto is the same as in the constructions of FIGURES 1 and 3.

The construction of FIGURE 5 is similar to that of FIGURE 3 except that the driving ring 80 has been split into two portions 80a and 80b. The portion 80a has a length preferably not exceeding the axial dimension of the pole 42. It is normally disposed with half its length within the portion of the gap 24) between the poles 42 and 46. The portion 8%, similar to the portion Silo.

is disposed halfway into the portion of the gap 20 between the poles 44 and 46.

Normally, there are attractive forces tending to pull both the portions 80a and 80b further into the gap 20. However, in the absence of current in the voice coil 34 of FIGURE 5, these forces are equal, and the tubular portion 22a of the diaphragm 22 remains as shown. When current is passed through the voice coil, there is a difference between the magnetic field strengths in the portions of the gap 29 adjacent to the poles 42 and 44. Accordingly, a greater force is exerted on one of the portions of the driving ring than on the other portion, and motion of the ring and the diaphragm attached thereto ensues. Reversal of voice coil current results in reversal of the direction of motion, and thus acoustical output energy is delivered from the diaphragm 22 in response to the application of alternating-current signals to the voice coil 34.

Thus, I have described a new loudspeaker provided with a stationary voice coil and using a magnetic driving member attached to the diaphragm to generate the acoustical output energy of the speaker. The driving ring is disposed in the gap of the static magnetic field circuit, and it is subjected to spatial variations in the static field distribution resulting from current through the voice coil. A force is exerted on the driving ring in response to these variations, and diaphragm motion follows this force. The loudspeaker can be fabricated at low cost, particularly the embodiment of FIGURES 1 and 2, in which the extra pole structure in which the voice coil is enclosed may be an integral part of a stamping forming the frame 24.

The speaker also has a high efficiency as well as faithfulness of reproduction. The high efficiency permits the use of lower amplifier power than heretofore practicable. Also, since it corresponds to closer coupling between the mechanical-acoustical system and the electrical system of the transducer, further improvement in quality of reproduction is possible. Variations in the load will in large part be reflected back into the voice coil. The coil may thus be connected in a feedback loop in the amplifier to minimize the efiects of the variations.

It will thus be seen that the objects set forth above, among those made apparent from the preceding descrip tion, are efficiently attained and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

I claim:

1. An electromechanical transducer comprising, in combination, a stationary magnetic structure having a generally E-shaped cross section and including a field magnet projecting a field between the inner arm of said E serving as one pole and the outer arms serving as the other pole, means forming a cavity in a first one of said poles opening toward the other pole, a coil providing a difference in magnetic potential between the areas along said first pole on opposite sides of said cavity in response to current through said coil, a driving member disposed symmetrically in the space between said poles, said driving member having a pair of spaced apart portions of high permeability disposed partially opposite each of said respective areas, each of said high permeability portions describing a closed path around said inner arm, each of said high permeability portions comprising parts of magnetic material spaced apart along said path by parts of non-magnetic material in such manner as to interrupt the magnetic paths between said inner and outer poles by Way of said portions having the shortest air gap lengths when said portions are displaced from their equilibrium positions between said poles.

2. The combination defined in claim 1 in which said paths are circular and said inner arm has a second cross section which is circular to conform to said path.

3. An electroacoustical transducer comprising, in combination, a stationary magnetic structure having a generally E-shaped cross section and including a field magnet projecting a field between the inner arm of said E serving as one pole and the outer arms serving as the other pole, means forming a slot in a first one of said poles opening toward the other pole, a coil providing a difference in magnetic potential between the areas along said first pole on opposite sides of said slot in response to current through said coil, a driving member disposed symmetrically in the space between said poles, said driving member having a pair of spaced apart portions of high permeability disposed partially opposite each of said respective areas, each of said high permeability portions describing a closed path around said inner arm, each of said high permeability portions comprising parts of magnetic material spaced apart along said path by part of non-magnetic material in such manner as to interrupt the magnetic paths between said poles by way of said portions having the shortest air gap lengths when said portions are displaced from their equilibrium positions between said poles, and a diaphragm attached to said driving member for movement therewith along said inner arm.

4. The combination defined in claim 3 in which said paths are circular and said inner arm has a second cross section which is circular to conform to said path.

References Cited in the file of this patent UNITED STATES PATENTS 540,969 Field June 11, 1895 1,577,254 Hayes Mar. 16, 1926 1,643,169 Nyman Sept. 20, 1927 1,683,945 Baldwin Sept. 11, 1928 1,732,644 Farrand Oct. 22, 1929 1,743,749 Arkell Jan. 14, 1930 1,763,846 Fay June 17, 1930 1,784,517 Farrand Dec. 9 1930 1,797,965 Peterson Mar. 24, 1931 1,806,914 Peterson May 26, 1931 1,823,603 Haynes Sept. 15, 1931 1,878,919 Van Dam et a1 Sept. 20, 1932 1,899,561 Dorsey Feb. 28, 1933 1,918,995 Ten Pas July 18, 1933 1,923,959 Williams Aug. 22, 1933 2,026,994 Messick Jan. 7, 1936 2,073,561 McCracken Mar. 9, 1937 2,110,193 Best Mar. 8, 1938 2,405,185 Benioff Aug. 6, 1946 2,488,734 Mueller Nov. 22, 1949 2,494,918 Volkers Jan. 17, 1950 FOREIGN PATENTS 150,322 Great Britain Nov. 21, 1921 

1. AN ELECTROMECHANICAL TRANSDUCER COMPRISING, IN COMBINATION, A STATIONARY MAGNETIC STRUCTURE HAVING A GENERALLY E-SHAPED CROSS SECTION AND INCLUDING A FIELD MAGNET PROJECTING A FIELD BETWEEN THE INNER ARM OF SAID E SERVING AS ONE POLE AND THE OUTER ARMS SERVING AS THE OTHER POLE, MEANS FORMING A CAVITY IN A FIRST ONE OF SAID POLES OPENING TOWARD THE OTHER POLE, A COIL PROVIDING A DIFFERENCE IN MAGNETIC POTENTIAL BETWEEN THE AREAS ALONG SAID FIRST POLE ON OPPOSITE SIDES OF SAID CAVITY IN RESPONSE TO CURRENT THROUGH SAID COIL, A DRIVING MEMBER DISPOSED SYMMETRICALLY IN THE SPACE BETWEEN SAID POLES, SAID DRIVING MEMBER HAVING A PAIR OF SPACED APART PORTIONS OF HIGH PERMEABILITY DISPOSED PARTIALLY OPPOSITE EACH OF SAID RESPECTIVE AREAS, EACH OF SAID HIGH PERMEABILITY PORTIONS DESCRIBING A CLOSED PATH AROUND SAID INNER ARM, EACH OF SAID HIGH PERMEABILITY PORTIONS COMPRISING PARTS OF MAGNETIC MATERIAL SPACED APART ALONG SAID PATH BY PARTS OF NON-MAGNETIC MATERIAL IN SUCH MANNER AS TO INTERRUPT THE MAGNETIC PATHS BETWEEN SAID INNER AND OUTER POLES BY WAY OF SAID PORTIONS HAVING THE SHORTEST AIR GAP LENGTHS WHEN SAID PORTIONS ARE DISPLACED FROM THEIR EQUILIBRIUM POSITIONS BETWEEN SAID POLES. 